1. Field of the Invention
The present invention relates to a method for distributing contention among terminals in a contention access period of a superframe in regard to a Medium (or Media) Access Control (MAC) layer employing a beacon-enabled mode in IEEE 802.15.4 standard.
2. Description of the Prior Art
A Personal Area network (PAN) corresponds to a concept which contrasts with the widely known Local Area Network (LAN) or Wide Area Network (WAN), and implies that every individual has their own unique network. Namely, a PAN refers to a single network constructed by terminals owned by a single person for the sake of that person's convenience. The concept of constructing a PAN by wireless means as described above corresponds to a Wireless Personal Area Network (WPAN).
In an effort to wirelessly implement a PAN, IEEE 802.15 working group has determined the WPAN as a standard for short-distance wireless networks, and has four Task Groups (TGs) for the IEEE 802.15 working group to administer. IEEE 802.15.1 corresponds to the very well-known Bluetooth, IEEE 802.15.3 and IEEE 802.15.3a correspond to a high-rate WPAN, and IEEE 802.15.4 having the alias “ZigBee” performs a standard job on a low-rate WPAN below 205 kbps.
The IEEE 802.15.4 protocol prescribes a PHYsical (PHY) layer and a Medium (or Media) Access Control (MAC) layer as standards regarding a low-rate wireless PAN. The MAC layer supports a beacon-enabled mode and a beacon-disabled mode. In the beacon-enabled mode, a superframe is prescribed, so as to define a time boundary in accordance with a media access method. A superframe includes a Contention Access Period (CAP), a Contention Free Period (CFP) and an inactive period, end the length of each period is determined by a beacon frame.
FIG. 1 illustrates a structure of a conventional superframe regarding the MAC layer using the beacon-enabled mode in the IEEE 802.15.4 standard. A terminal uses a slotted Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) scheme to gain access to a medium in the CAP of the superframe.
When the terminal has data to be transferred, the terminal first checks through a Clear Channel Assessment (CCA) if the medium is being used by other terminals. If it is found through the first CCA that the medium is not being used by other terminals, the terminal performs the second CCA. At this time, if it is found through the second CCA that the medium is not being used by other terminals, the terminal transfers a frame.
If it is found through a CCA that the medium is being used by other terminals, the relevant terminal performs a backoff process where the relevant terminal waits for an arbitrary period of time, and then performs a CCA again.
The backoff process is as follows. A terminal attempting an initial transfer sets a Backoff Exponent (BE) to macMinBE, sets a Content Window (CW) to ‘2’, initializes the Number of Backoffs (NB) to ‘0’, and then selects an optional backoff counter in the range of [0, 2BE −1]. Every time a unit backoff period (i.e., aUnitBackoffPeriod) expires, the terminal reduces the value of the backoff counter by one. Then, a terminal having a backoff counter whose value equals ‘0’ at a starting time point of each unit backoff period performs a CCA. One unit backoff period corresponds to 20 symbols, and time regarding a unit symbol is determined by a relevant PHY layer.
If the terminal fails to perform the CCA, the terminal initialises the CW to ‘2’, and then increases both the BE and the NB by one. The BE has aMaxBE as the upper limit. If the NB is smaller than macMaxCSMABackoffs following an increase thereof, the terminal selects an optional backoff counter in the range of [0, 2BE −1], and then performs a backoff process again. If the NB is larger than macMaxCSMABackoffs, it is determined that the transfer of a relevant frame has failed, and the BE, the CW and the NB are initialised to macMinBS, ‘2’, and zero, respectively.
FIG. 2 shows a normalized saturation throughput of a slotted CSMA/CA scheme in accordance with an increase in the number of terminals sharing a medium. If multiple terminals simultaneously transfer frames, collisions occur among the transferred frames, and the more terminals, the larger the probability of the occurrence of collisions. Also, as the number of terminals increases, the period of time for which the terminals possess a medium becomes longer, the probability of failure in a CCA rises, and consequently, the number of transfers which are subject to failure becomes greater. If these factors cause a rise in the number of terminals, the total throughput decreases.